This invention relates to a composite plate suitable for use as a structural plate, such as a floor plate, a wall plate, a ceiling plate, etc., of vehicles of subways and other means of transportation.
Heretofore, it has been usual practice to render plywood plates difficult to burn by impregnating them with a flame-proof agent. However, even if an attemt was made to render the plywood plates difficult to burn by impregnating them with a flame-proof agent, it has been difficult to achieve excellent results. Although it is possible to suppress flame formation in the event of a fire by using flame-proof plywood plates, it has been impossible to prevent smoke generation which is an important problem to be obviated in case of a fire. Meanwhile, attempts have been made to provide fire-proof plywood plates by attaching a stainless steel plate or an aluminum plate to each of top and bottom surface of an ordinary plywood or fire-proof plywood base plate. This type of fire-proof composite plate has had the disadvantage that, since material of the plywood base plate is exposed on all four sides, the plywood base plate might catch fire at its sides, making it impossible to render it completely fire-proof. An additional problem raised with regard to this type of fire-proof composite plate is that the plywood base plate might become damp due to the invasion of moisture through its exposed sides, causing corrosion to take place in the material or impairing dimensional stability of the plywood base plate.